The present invention relates to a combustion gas turbine and, specifically, it relates to an air-cooled exhaust casing of a gas turbine provided with cooling air passages in the wall of the casing.
In a gas turbine, a mixture of a high-pressure combustion air and fuel are burned in combustors in order to generate high-pressure and high-temperature combustion gas. This combustion gas is supplied to a plurality of turbine stages, each comprising stator blades and rotor blades and disposed in a turbine casing, in order to convert the energy of the combustion gas into the rotational energy of the rotor blades. The combustion gas, the pressure thereof being lowered after passing the turbine stages, passes through a diffuser disposed in an exhaust casing in order to recover its pressure before it is discharged to the atmosphere.
Usually, the stator blades of the respective turbine stages are fixed to a stator ring disposed in the turbine casing. A predetermined amount of gap is maintained between the outer surface of the stator ring and the inner wall of the turbine casing so that an annular cavity is formed between each of the stator rings and the inner wall of the turbine casing.
Cooling air is supplied from the gas turbine air compressor into the annular cavities between the stator rings and the turbine casing to prevent the stator rings and the turbine casing wall from being excessively heated by high-temperature combustion gas passing through the respective turbine stages.
The annular cavities between the stator rings and the turbine casing wall are arranged in the axial direction and are separated from each other by diaphragms. Cooling air supplied to each annular cavity enters cooling air passages formed in the stator blades through the stator ring and, after cooling the stator blades, discharged into the combustion air passages to thereby mix with combustion gas.
The exhaust casing is connected to the downstream end of the turbine casing and, as the combustion gas flows into the exhaust casing after expanding at the respective turbine stages, the temperature of the exhaust gas flowing into the exhaust casing is relatively low. Further, since a diffuser is disposed in the exhaust casing and the inner wall of the exhaust casing is shielded from the exhaust gas by the diffuser, the temperature of the exhaust casing wall is substantially lower than that of the turbine casing wall. Therefore, no cooling facility is provided in the exhaust casing wall.
However, it has been found that the conventional cooling system of the turbine casing as explained above, in which cooling air is supplied to the annular cavities for cooling the casing and stator blades, sometimes causes problems such as a damage or a distortion of the exhaust casing connected to the turbine casing, or a distortion of the stator rings.
As explained above, cooling air supplied to the annular cavities of the respective turbine stages is discharged into the combustion gas passing through the respective turbine stages after cooling the stator rings and the stator blades of the respective stages. Therefore, the pressure of cooling air supplied to the cavities must be higher in the upstream turbine stages. Usually, cooling air is extracted from the gas turbine air compressor discharge or from the intermediate stages of the compressor. Since the pressure requirement for the cooling air is different in the respective cavities, cooling air supplied to the cavities of upstream stages, for example, is taken from the discharge or high-pressure stages of the gas turbine air compressor.
Similarly, cooling air for the cavities of the intermediate turbine stage is extracted from intermediate-pressure stage of the compressor and cooling air for the cavities of the downstream turbine stages is taken extracted from low-pressure stages of the compressor.
The temperature of the air extracted from the compressor stages becomes higher as the pressure of the extracted air becomes higher. Therefore, when the air extracted from the gas turbine compressor is used for cooling air, the temperature of the cooling air supplied to the upstream turbine stages becomes higher than the temperature of the cooling air supplied to the downstream turbine stages and, at the most downstream (last) turbine stage, i.e., at the turbine stage nearest to the joint where the exhaust casing is connected, the cooling air supply temperature becomes the lowest. This causes the wall temperature of the turbine casing near the exhaust casing joint to be considerably lower than the combustion gas (exhaust gas) temperature.
On the other hand, no cooling facility is provided on the exhaust casing. Therefore, though the wall of the exhaust casing is shielded from the hot exhaust gas by the diffuser disposed in the exhaust casing, the metal temperature of the exhaust casing approaches that of the exhaust gas due to the radiation from the diffuser.
Therefore, when an air cooling system of the turbine casing is used, the metal temperature of the turbine casing wall becomes relatively low although the metal temperature of the exhaust casing wall becomes relatively high. Therefore, a large temperature difference occurs between the turbine casing and the exhaust casing at the joint portion therebetween. This large temperature difference generates a relatively large thermal stress in the exhaust casing. In general, the exhaust casing has a sufficient rigidity to withstand such a thermal stress and the thermal stress does not cause immediate damage. However, when the gas turbine is operated for a long period in the condition where the exhaust casing is subject to a large thermal stress, damage such as cracking due to a low cycle fatigue, or a deformation, may occur in the exhaust casing.
Further, the stator rings are disposed in the annular cavities in order to hold the turbine stator blade. In the last turbine stage, the outer side (the cavity side) of the stator ring is cooled by the low temperature cooling air and the inner side (the hot gas side) of the stator ring contact with the hot exhaust gas, thereby the temperature difference across the stator ring in the radial direction becomes very large in the last turbine stage. Therefore, a large thermal stress is generated in the stator ring of the last turbine stage due to the temperature difference and, in some extreme case, a damage or distortion of the stator ring occurs due to the large thermal stress.
In view of the problems in the related art as set forth above, the object of the present invention is to provide an air-cooling system for a gas turbine casings capable of preventing generation of a large thermal stress on the exhaust casing and turbine stator rings.
According to the present invention, there is provided an air-cooled gas turbine exhaust casing containing an exhaust diffuser and connected to a turbine casing having turbine stages each including turbine stator blades and rotor blades, the exhaust casing being provided with cooling air passages disposed within the casing wall and extending in the axial direction, wherein the cooling air passages include cooling air inlets disposed near the downstream end of the exhaust casing, cooling air outlets disposed near the upstream end of the exhaust casing and connected to an annular cavity formed between a stator ring holding the stator blades of the last turbine stage and the inner surface of the turbine casing wall, whereby cooling air enters into the cooling air passages at the portion near the downstream end of the exhaust casing, flows within the exhaust casing wall toward the upstream end of the exhaust casing and enters the annular cavity of the last turbine stage from the downstream end of the turbine casing.
According to the present invention, cooling air is supplied to the cooling air passages of the exhaust casing from the portion near the downstream end of the exhaust casing. This cooling air passes through the cooling air passages in the exhaust casing wall toward the upstream end of the exhaust casing. Further, the cooling air after passing through the cooling air passages in the exhaust casing is supplied to the annular cavity of the last turbine stage from the upstream end of the exhaust casing.
Thus, the temperature of the exhaust casing wall near the connection to the turbine casing becomes lower than that of the conventional non-cooled exhaust casing. Further, since cooling air is supplied to the annular cavity of the last turbine stage after being warmed in the cooling air passage of the exhaust casing, the wall temperature of the turbine casing at the portion of the last turbine stage (i.e., the portion near the joint to the exhaust casing) becomes higher than that of the conventional case. Therefore, the temperature difference between the turbine casing and the exhaust casing, as well as the resultant thermal stress in the exhaust casing, becomes very small in the present invention.
Further, as the temperature of the cavity side of the stator ring becomes higher than the conventional case in the present invention, the temperature difference between the cavity side and the hot gas side of the stator ring is also reduced. Thus, according to the present invention, the thermal stress exerted on the stator ring is largely reduced.
In the present invention, although the exhaust casing is air-cooled in addition to the turbine casing, it is possible to suppress the increase in the consumption of the cooling air. In the conventional cooling system, pressurized air extracted from an intermediate compressor stage of the gas turbine air compressor is used as cooling air supplied to the annular cavity of the last turbine stage. Therefore, if the same air, i.e., air extracted from the intermediate compressor stage is supplied to the cooling air passage of the exhaust casing, the additional air-cooling of the exhaust casing can be carried out without increasing the cooling air consumption in the present invention. Thus, according to the present invention, the temperature difference and the resultant thermal stress in the exhaust casing can be reduced without increasing the cooling air consumption that deteriorates the thermal efficiency of the gas turbine.